1. Chapter 24 Weather The state of the atmosphere at any given time or place. Differences in air pressure are caused by unequal heating of Earth’s surface. The region along the equator receives more solar energy than the poles. This creates regions of low pressure and high pressure which creates winds. Air moves from areas of high pressure to areas of low pressure = wind. In general, air moves from the equator to the poles. The processes that affect air movement also influence weather.

2. Air Masses An air mass is a large body of air in the lower troposphere that has similar characteristics of temperature and humidity. Air masses move across the earth’s surface, causing changes to the weather. The boundary between air masses is called a front (frontal system). Approaching fronts change the weather.

3. The four types of air masses that affect the weather of North America come from six regions.

4. Air masses are based on where they come from: 1.Maritime - moist – from the ocean 2.Continental - dry – from the land 3.Tropical - warmer – from near the tropics 4.Polar - colder – from near the poles An air mass usually brings the weather of its source region: mT – maritime tropical – higher humidity and warmer cT – continental tropical – lower humidity and warmer mP – maritime polar – higher humidity and cooler cP - continental polar – lower humidity and cooler

5. Fronts A boundary that forms between two different air masses. When two unlike air masses meet, density differences usually keep the air masses separate. A cool air mass is dense and does not mix with the less dense air of a warm air mass. No fronts in the tropics. Why? Similar air mass densities Air masses that don’t differ in pressure, don’t move much. For a front to form, one air mass must collide with another air mass Four different kinds of fronts: 1.COLD FRONT 2. WARM FRONT 3. STATIONARY FRONT 4. OCCLUDED FRONT

6. Squall Line

7. Cold Front

8. Warm front

9. Stationary front

10. Occluded front

11. Plotting Fronts and Precipitation Most weather maps mark the locations of fronts and areas of precipitation. – Fronts are identified by sharp changes in wind speed, wind direction, temperature, and humidity. – Areas of precipitation are commonly marked by using colors or symbols. Light rain = green Snow = grey or white Different colors also may represent different amounts of precipitation.

12. FRONTS ARE ONLY ATTACHED TO LOWS.

13. Plotting Pressure Lines on weather maps connect points of equal measurement. – Equal temperatures = isotherms – Equal atmospheric pressures = isobars Closely spaced bars indicate rapid change in pressure and high wind speeds. Widely spaced bars indicate gradual change in pressure and low wind speeds. Isobars that form circles indicate centers of high (H) or low (L) pressure.

14. Isobars

15. Winds Isobars connect points of equal pressure. Winds blow parallel to isobars. In the U.S. winds flow clockwise around highs. In the U.S. winds flow counterclockwise around lows.

17. Mid – Latitude Cyclones Last several days (formation to dissipation) – Areas of low pressure that are characterized by rotating wind that moves toward the rising air of the central low pressure region. – Influence weather patterns in the middle latitudes. In North America, travel 45 km/h from west to east, spin counterclockwise.

20. Anticyclones Air sinks and flows outward from a center of high pressure. Clockwise air movement in North America (Coriolus Effect) Bring dry weather, sinking air does not form clouds. If become stationary, may create pollution problems and/or drought.

21. HIGH OR LOW? CYCLONIC OR ANTICYCLONIC ?

22. Severe Weather Any weather that may cause property damage or loss of life. – THUNDERSTORMS- WINTER STORMS – HURRICANES- TORNADOES WATCH: Conditions are ideal for severe weather to develop. WARNING: Severe weather has been spotted or is expected within 24 hours.

23. Types of Forecasts Nowcasts – Use radar to focus on timing precipitation and tracking severe weather. Daily forecasts – Predict weather conditions for a 48-hr period. Extended forecasts – look ahead 3-7 days Medium range forecasts – look ahead 8-14 days – Use computer analysis Long range forecasts – cover monthly and seasonal periods – Use computer analysis Accurate forecasts are good for 0 to 7 days but accuracy decreases with each day.

24. Thunderstorms Form in cumulonimbus clouds. Produce intense precipitation, lightning, and strong winds. Typically short-lived, usually 15-30 minutes. Generally form along fronts on hot humid summer days. Three distinct stages: – Cumulus Stage Warm, moist air rises; cumulus clouds form. – Mature Stage Condensation continues while clouds keep building Heavy rain/Hail occurs and downdrafts form – Dissipating Stage Downdrafts stop air currents from rising Storm eventually fades

26. Lightning and Thunder During thunderstorms, clouds discharge electricity in the form of lightning. The upper part of the cloud  + charges Lower part of the cloud  - charges Lightning is a huge spark that travels within a cloud, from cloud to cloud, or from cloud to the ground. It can travel miles from the actual storm. There is no such thing as “heat” lightning. Temperatures can reach 54,000 o F To find the distance to the storm, count the number of seconds from the time of the lightning flash to when thunder is heard. Divide this number by 5. The released electricity heats the air, and the air expands rapidly which produces a loud noise – thunder

28. Tornadoes Smallest, most violent, shortest-lived severe storm. Spiral around low pressure areas. A destructive, rotating column of air with very high wind speeds. Forms when a thunderstorm meets high-altitude, horizontal winds which cause the rising air in the thunderstorm to rotate. Starts out as a rapidly spinning, funnel-shaped extension from the base of the cumulonimbus cloud. These narrow (100m wide) funnel shaped extensions may or may not touch ground. Called tornadoes when they touch the ground. Destructive force is due to the speed of the winds in the tunnel. Winds may reach 400 km/hr Most deaths and injuries are from flying objects or collapsing buildings.

29. Tornado Alley

31. Scale for Measuring Tornadoes Link

32. Hurricanes Tropical storms that form over warm, tropical ocean water. The warm ocean heat increases the force of the rising air. Winds spiral inwards toward intense low pressure. Wind speeds are greater than 74 km/h Consists of a series of thick cumulonimbus cloud bands that spiral upward around the center (eye) of the storm. Winds increase toward the eye and may reach 275 km/h along the eyewall. The eye is an area of calm, clear, sinking air. Typically can reach 700 km in diameter Dangerous due to rising sea levels and large waves (storm surge) Most deaths due to drowning.

33. Link

34. Saffir-Simpson Scale for Hurricanes Based on central pressure, wind speed, and storm surge

35. Hurricane season in the Atlantic begins June 1 st and ends November 30th Hurricanes are not given names, tropical storms are given names, and they retain their name if they develop into a hurricane. Tropical depression Tropical storm Hurricane

36. Global Weather Monitoring Weather stations around the world. – manned and automated. – Info sent to collection centers. – WMO (World Meteorological Organization) (1873). – sponsors World Weather Watch (rapid exchange of info.) now part of the United Nations 1970

38. Air Temperature:60 FDew Point Temperature: 45 F Wind Speed: 15 mphWind Direction: Southwest Precipitation/Weather: Light RainAtmospheric Pressure: 1008 mb Cloud Cover: 70%Cloud Type: Stratus